1. Field of the Invention
This invention relates to the production of the biomass of aerobic granules for the treatment of waste water.
In the treatment of waste waters by oxidative biological purification in aerobic granular sludge blanket (AGSB) reactors, waste water flows in an upward direction through an oxidation chamber in which micro-organisms are present. Movement of the suspension of waste water and micro-organisms within the waste water chamber is provided by the introduction of an oxygen-containing gas which also serves to mix the suspension of biological material and waste water. Within the reactors are inner zones of regulated settling which cooperate in the removal or accumulation of granules of a specifed size range.
Other types of oxidative reactors which rely on the introduction of an oxygen-containing gas at the bottom of an oxidation chamber doubling up through waste water to be treated are generally referred to as uplift reactors. Such reactors are generally seeded with an active biological sludge which consumes contaminants within the waste water thus reducing the level of those contaminants and producing biomass which can later be removed from the reactor for recycle, or other uses, or disposal.
One such reactor is disclosed in U.S. Pat. No. 5,985,150 assigned to Biothane Systems International B.V. In this patent, there is disclosed an uplift reactor in which unsupported granular sludge is used to treat waste water. In this reference the sludge is introduced either as a supported or unsupported sludge into the reactor and waste water and an oxygen-containing gas supplied through the bottom of the reactor. The granular sludge is carried with the upwardly flowing gas into a settling region where a portion of the waste water is separated from the sludge and the remaining suspension returned to the oxidation chamber.
One problem with oxidative waste water treatment in reactors of this type is the lack of cohesion between the unsupported biomass thus making the handling of the biomass generally difficult. In particular, removing the biomass from the treated waste water and producing biomass which is suitably robust to be used as a seed for other reactors, has proved to be difficult.
However despite the difficulties in handling unsupported aerobic biomass granules it is recognised that the use of such granules over supported biomass granules generally has the potential to improve the purification efficiency of reactors thus allowing the use of smaller reactor systems. If the biomass of aerobic granules can also be produced to a commercially acceptable level, it is expected that the use will reduce suspension and mixing energy requirements and give rise to less erosion of equipment. However it has been difficult to produce the biomass of aerobic granules or biogranules of a sufficient size with adequate physical properties to be used effectively in aerobic uplift reactors.
It is an object of the present invention to provide a method of production of aerobic biogranules which can be used in the treatment of waste water.
The present invention is directed to a method of producing aerobic granules under controlled conditions for the treatment of waste water. The method comprising the steps of:
a) introducing waste water into a reactor containing an active biomass;
b) supplying an oxygen-containing gas to the reactor to provide a mixing action and transfer of dissolved oxygen into the sludge in the waste water, the supply of oxygen-containing gas providing a superficial upflow gas velocity above 0.25 cm/s;
c) initiating a period of nutrient starvation in the reactor while continuing to supply oxygen-containing gas;
d) allowing formed aerobic granules to settle;
e) discharging and replacing at least a portion of the waste water;
f) repeating steps (a), (b), (c), (d) and (e) until the granules have predetermined physical properties; and
g) recovering biomass granules within those predetermined properties.
The applicants have found that aerobic granules suitable for uplift reactors and particular AGSB reactors may be formed under the above conditions without the need for the use of carrier materials.
It is believed that the starvation period in the reactor under shear conditions causes the biomass to agglomerate forming much more robust granules.
In another aspect of the invention, there is provided aerobic biogranules for waste water treatment having physical characteristics suitable for use in uplift reactors. Preferably these physical characteristics include a roundness aspect between 0.0 to 3.0, density between 1.004 to 1.85 g/cm3, and average particle size between 100 and 10,000 xcexcm.
Preferably the distance from any position within the biogranules to the outer surface of the biogranules is less than 800 xcexcm. Thus for perfectly spherical biogranules the maximum diameter is 1600 xcexcm.